Evolution, paleoecology and sequence architecture of an ... · the Early Cenozoic large benthic foraminifera, Nummulites were unique in their rock-forming ... PTC-CCS = Paleo-Tethyan

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GeoArabia, 2013, v. 18, no. 4, p. 49-80Gulf PetroLink, Bahrain

Evolution, paleoecology and sequence architecture of an Eocene carbonate ramp, southeast Zagros Basin, Iran

Afshin Zohdi, Reza Mousavi-Harami, Seyed Ali Moallemi, Asadollah Mahboubi and Adrian Immenhauser

ABSTRACT

We document and discuss the results of detailed fieldwork, facies analysis and the subsequent integration of paleoecological evidence from the Middle Eocene carbonate ramp succession in the southeast Zagros Basin (Jahrum Formation). A combination of a sea-level fall and tectonic and diapiric basement uplift favored the initiation of the Jahrum carbonate platform. The lower portions are affected by pervasive, probably early diagenetic dolomitization, whilst the upper Jahrum consists mainly of limestone. Here, the focus is on the limestone portions of the Jahrum Formation. Based on the abundance, diversity and rapid evolutionary turnover of the alveolinids and nummulitids, the limestone intervals of the Jahrum Formation are interpreted for the upper Middle Eocene (Bartonian). The Jahrum Formation is capped by a major unconformity and overlain by the Lower Oligocene mixed clastic/carbonate Razak Formation.

Based on data from field sections, eight facies associations and a series of sub-types have been established, which correspond to inner-, middle- and outer-ramp depositional environments. In their overall context, these data show a southward-dipping inner-ramp-to-basin transect. Towards the Coastal Fars (e.g. Hulur-01 Well) the Jahrum grades laterally into deep-marine Pabdeh foredeep shale units. Based on facies analysis and paleoecological evidence from larger benthic foraminifera, a major transgressive-regressive pattern is recognized in all outcrop sections of the Jahrum. The lowermost stratigraphic units of the formation are here interpreted as a distally steepened ramp. Evidence comes from abundant allochthonous shallow-water facies in the distal, deeper-ramp setting. Shallow-water carbonate clasts were exported towards the basin, a feature that is probably linked to relative sea-level fall control. Furthermore, local to regional basement instabilities by salt diapir-related basement reorganization was arguably of significance. Upsection, evidence is found that the ramp system evolved from a distally steepened to a homoclinal geometry with an overall very gentle slope geometry during the Late Bartonian.

The data shown here are significant for those concerned with the Paleogene evolution of the southeast Zagros Basin and provide a well-exposed case example of a Middle Eocene carbonate ramp factory.

INTRODUCTION

The evolution of circum-Tethyan carbonate platform systems of the Early Paleogene greenhouse world has been the topic of intensive research over the last decades. The focus of previous studies included paleoenvironmental conditions (e.g. Moody, 1987; Eichenseer and Luterbacher, 1992; Jorry et al., 2003; Jorry et al., 2006; Beavington-Penney et al., 2006; Zohdi et al., 2011; Wilson et al., 2012; Ćosović et al., 2012), shallow benthic biostratigraphy (e.g. Bismuth and Bonnefous, 1981; Serra-Kiel et al., 1998; Boukhary, 2006; Hottinger, 2007; Saraswati et al., 2012; Whidden and Jones, 2012), orbital forcing and depositional cyclicity (Abu El Ghar, 2012), oil potential (e.g. Loucks et al., 1998; Racey, 2001; Vennin et al., 2003; Swei and Tucker, 2012) and the response of carbonate systems to long- and short-term paleoclimatic change (e.g. Scheibner et al., 2007; Scheibner and Speijer, 2008; Cotton and Pearson, 2011).

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During the Paleogene, shallow-water carbonates accumulated over large areas of the peri-Tethyan region, accounting for more than 80% of the global neritic carbonate production during Eocene times (Philip, 2003). In the Paleogene world, larger benthic foraminifera were of significance as major platform sediment contributors in nearly all circum-Tethyan shallow-marine environments. Amongst the Early Cenozoic large benthic foraminifera, Nummulites were unique in their rock-forming potential. Nummulitic foraminifera are abundant in Paleocene to upper Eocene sedimentary rocks of the Mediterranean realm and the Arabian Peninsula. Nummulitic limestones form hydrocarbon reservoirs in offshore North Africa and India and are potential exploration targets in the Middle East (Beavington-Penney et al., 2005). Large benthic foraminifera facies of this age are generally associated with carbonate ramp settings (Buxton and Pedley, 1989). The relative distribution of different foraminifera across carbonate ramps is used as a valuable tool in paleoenvironmental and paleobathymetric reconstructions (Racey, 2001; Beavington-Penney et al., 2006; Jorry et al., 2006; Barattolo et al., 2007).

In the Paleogene of the Zagros sedimentary basin of Iran, these carbonate deposits, mainly referred to as Jahrum Formation, are present in numerous well-exposed outcrops and provide an excellent natural laboratory. In the type locality (north flank of Kuh-e Jahrum in Fars Province; see Figure 31 in James and Wynd, 1965) and many other exposures, the lower parts of the Jahrum Formation are pervasively dolomitized. The recognition of the pre-dolomitization facies is not without problems. Nevertheless, the middle and upper portions of the Jahrum Formation mainly consist of large benthic foraminifera limestones (e.g. Nummulites, Discocylina, Alveolina, Somalina and Orbitolites). In addition, abundant coarse-grained carbonate breccia intervals and re-sedimented shallow-water facies in proximal domains of the Jahrum Formation are present and their origin and significance is discussed here. The compilation of a detailed biostratigraphic framework allowed for a detailed correlation between spatially separated sections. Time correlation allows for an assessment of changes in platform/ramp architecture and facies and biotic changes from proximal to distal settings.

Published evidence on the Jahrum Formation is scarce (e.g. Nadjafi et al., 2004; Taheri et al., 2008; Moallemi, 2009). Specifically, a detailed sedimentological and sequence-stratigraphic interpretation was lacking. In addition, the regional reconstruction of the overall depositional architecture and the correlation of stratigraphic units based on a rigorous biostratigraphic framework such as shown here is novel. Furthermore, this work has applied significance as the Jahrum Formation constitutes a major gas reservoir.

The present paper is, to the knowledge of the authors, the first openly accessible, detailed data set of the Jahrum Formation. This lack of data represents a major obstacle for those concerned with the Paleogene evolution of carbonate ramp systems in the Coastal Fars. Consequently, the goals of this paper are: (1) to document and discuss a detailed facies analysis of the Jahrum Formation ramp in its spatial and bathymetric context; (2) to present a tentative first sequence architecture model; and (3) to provide an interpretation of the changes in depositional regime and facies with time and under changing tectono-sedimentary boundary conditions.

REGIONAL GEOLOGICAL SETTING AND STUDY AREAS

Regional Geological Setting

The Zagros Orogen, forming a portion of the Alpine-Himalaya mountain chain, is a well-defined active doubly-vergent and asymmetric orogenic belt (Alavi, 2004). The northwestern boundary of the orogen is located along the East Anatolian strike-slip fault (Figure 1) in southeastern Turkey. The southeastern boundary of the Zagros Orogen is represented by the Oman Line (Falcon, 1974).

The Zagros sedimentary basin is the result of the Neogene collision between the Arabian and Iranian plates (Stöcklin, 1968; Alavi, 2007). The geotectonic evolution of the Zagros Orogen can be divided into four principal phases: (1) rifting in Permo-Triassic time and formation of the Neo-Tethyan ocean between the Iranian plates and Arabian plate; (2) subduction of the Neo-Tethyan oceanic plate beneath


Eocene sequence architecture southeast Zagros Basin, Iran

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the Iranian plate during Cretaceous times; (3) emplacement (“obduction”) of a number of Neo-Tethyan ophiolites over the Afro-Arabian passive continental margin in Late Cretaceous (Turonian to Campanian; Gnos et al., 1997; Schreurs and Immenhauser, 1999) time; and (4) collision of the Afro-Arabian continental lithosphere with the Iranian plates during the Late Cretaceous and the Paleogene (Alavi, 2007).

The Zagros Orogen consists of three distinctive, parallel tectonic zones (Figure 1). To the northeast, the Uremiah-Dokhtar Magmatic Assemblage is formed by a relatively narrow (50–80 km), linear belt of intrusive and extrusive rocks (Falcon, 1974; Figure 1). To the southwest of the Uremiah-Dokhtar Magmatic Assemblage, the Zagros Imbricate Zone forms the core of the orogen and consists of thrust systems active from the Late Cretaceous to the Recent (Alavi, 2007). The Zagros Fold-and-Thrust Belt, with an average width of ca. 300 km, extends parallel to the Zagros Imbricate Zone and then stretches further to the southwest (Figure 1). In contrast to the Zagros Imbricate Zone, in which exposed structures are predominantly thrust faults, the Zagros Fold-and-Thrust Belt is distinguished by its long (up to ca. 150–200 km), NW-SE strike “whale-back” anticlines, which are spectacularly displayed on satellite images. The study area of this paper is located in the southeastern Zagros Fold-and-Thrust Belt (Figure 1). The Jahrum Formation, carbonates, the stratigraphic interval described here, was deposited during the fourth principal phase of Zagros evolution as discussed above.

Figure 1: Subdivisions of the Zagros Orogenic Belt. See inset for regional location. Study area shown as red rectangle. Hydrocarbon fields of the region, oil in green and gas in red, are shown (modified from Alavi, 2007). PTC-CCS = Paleo-Tethyan continent-continent collisional suture; UDMA = Uremiah-Dokhtar Magmatic Assemblage; ZFTB = Zagros Fold-and-Thrust Belt; ZIZ = Zagros Imbricate Zone; ZS = Zagros Suture; MAC = Makran Accretionary Prism.

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Study Areas: Southeast Zagros Fold-and-Thrust Belt

The southeastern Zagros Fold-and-Thrust Belt constitutes a major structural feature within the Alpine-Himalayan Orogen, marking the transition between the Zagros Collision Belt to the west and the Makran Accretionary Prism and Oman Mountains to the east (Figure 1). This tectonic unit is marked by a sharp change in trend and style of the tectonic structures. The structures in this region describe a tight re-entrant underlined by the arched shape of the Musandam Peninsula jutting out into the Strait of Hormuz (Molinaro et al., 2004).

In the southeastern Zagros Fold-and-Thrust Belt, anticlines are built most commonly by the competent limestone rocks of the Eocene Jahrum and the Lower–Middle Miocene Guri formations. In contrast, synclines are composed mainly of Miocene sandstone and marls (Aghajari and Mishan formations) and Pliocene–Pleistocene conglomerates (Bakhtyari Formation).

Numerous salt diapirs are visible in the southeast Zagros Fold-and-Thrust Belt (Ala, 1974; Edgell, 1996; Jahani et al., 2009) and seem to be located preferentially towards the periclinal terminations of folds (for example, Namak and Finu anticlines), or at junctions between faults (Figure 2). This might point to an intimate relationship between Paleogene and Neogene structures and salt diapirs in the Zagros Mountains (Molinaro et al., 2004).

Salt diapirs were already active prior to Zagros folding, either as emergent diapirs forming islands in Paleogene to Neogene seas, or as buried domes initiated at least since the Permian. At the initiation of the Zagros folding, diapirs had already been reactivated by earlier tectonic events and salt movement along faults resulting in evaporitic facies reaching the surface. The overall geotectonic setting clearly affected the evolution of the Paleogene carbonate platforms in Iran (Callot et al., 2007; Jahani et al., 2009).

Large-scale anticlines and synclines in the study area have mostly an EW-trending orientation, which is different from other parts of the Zagros Basin with NW-oriented structures trending parallel to the main Zagros Orogen (Molinaro et al., 2004). From north to south, exposures visited in the context of this study are located in the Finu Anticline (56°10É, 27°48Ń), the Faraghun Anticline (56°24É, 27°54Ń), the Khush Anticline (56°37É, 27°34Ń) and the Genow Anticline (56°12É, 27°28Ń) (Figure 2). The locations and roads leading to the measured sections are also shown in Figure 2.

METHODOLOGY

Fieldwork

At each study site (Figure 2), detailed stratigraphic sections were measured, sampled and described with respect to carbonate facies and biota. Laterally well-exposed outcrops permit the physical correlation between individual sedimentary units. In order to expand regional stratigraphic evidence to the south, the Hulur-01 Well in the Coastal Fars was used and correlated. Significant facies contacts and important stratigraphic horizons were mapped in the field using photomosaics, while facies characteristics, stratal relationships and sample locations were noted.

Laboratory Methods

The petrographic description is based on the thin section petrography of 1,047, out of the 1,450 collected samples, allowing for the determination of carbonate components, facies and environmental interpretations. The facies nomenclature follows the textural classification scheme of Dunham (1962). Age-diagnostic benthic foraminifera were used for the establishment of a detailed biostratigraphic framework and/or their relative abundances with respect to planktonic foraminifera were used to determine bathyal (beyond the zone of light penetration) versus neritic (or shelfal) environments and subdivisions thereof (Beavington-Penney and Racey, 2004). Large benthic foraminifera associations,



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their autochthonous or parautochthonous assemblages, test size and test shape variation of Nummulites and the ratio of megalospheric (A-form) to microspheric (B-form) Nummulites aided in constraining the paleobathymetry of the depositional setting. The authors made use of the rigorous works of Beavington-Penney and Racey (2004) and also Jorry et al. (2006) in this respect.

The composition of associated fauna and non-skeletal grains was considered. Sedimentologic texture and structure including cross-bedding, boring, burrowing and encrustation have been described in a semi-quantitative manner.


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Sequence-Stratigraphic Framework

This study used the identification of a maximum flooding surface from outcrop data to sub-divide the Jahrum Formation into isochronous packages of sediment, or genetic stratigraphic sequences. In the case of the Jahrum Formation, the transition from retrogradation to progradational stacking patterns is commonly not expressed by a physically distinct surface, i.e. a marine hardground, but is rather represented by the most distal facies, here referred to as “maximum flooding interval” (MFI). This interval was assumed to be approximately contemporaneous and was therefore used as a datum from which to correlate the other measured depositional sequences.

With respect to the Jahrum Formation, the distribution, functional morphology and habitat of foraminifera was useful in order to determine deepening and shallowing trends in sedimentary sequences. To define depositional sequences, sequence boundaries formed during successive maximum regression were taken into consideration. The correlation of our data with the Arabian Plate sequence stratigraphy of Sharland et al. (2001), and the relative sea-level chart of Haq et al. (1988) and other workers closes an important gap in the overall stratigraphic framework of the southeast Zagros Basin.

LITHOSTRATIGRAPHY

In the study area, the Jahrum Formation is characterized by gray (to brown when weathered), cliff-forming, medium to very thickly-bedded dolostone, dolomitic limestone and limestone. Carbonate sequences of the Jahrum Formation consist mainly of large benthic foraminifera (nummulitids, alveolinids, soritids and miliolids), along with other shallow-water skeletal (e.g. algae and bivalve) and non-skeletal components. In all studied outcrops, the formation is overlain by red sandstones and conglomerates of the Razak Formation. In the northern outcrops (Finu and Faraghun anticlines), the Jahrum Formation conformably overlies the marly limestones of the Gurpi Formation and in the southern outcrops, the Jahrum Formation overlies the shale deposits of the Pabdeh Formation (Figure 3).

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In the northern portions of the southeast Zagros Basin (e.g. Finu Anticline), the Jahrum Formation reaches a cumulative thickness of about 600 m and can be divided into two major units (Figure 4a): (1) A lower unit, reaching a stratigraphic thickness of about 300 m and consisting mostly of medium-bedded dolostone; and (2) an upper unit composed of dolomitic limestone and very thickly-bedded limestone with abundant shallow-water benthic foraminifera (Alveolina, Orbitolites and Somalina). The upper unit reaches ca. 200 to 300 m in stratigraphic thickness.

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Figure 4: Outcrop views of Jahrum Formation in (a) northern (564 section meter) and (b) southern outcrop (290 section meter) belts. At the north of the studied area the Jahrum Formation can be divided into two major units (including dolostone and limestone). But, toward to south dolomitization of the lower Jahrum did not take place.


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Towards the south (e.g. Khush Anticline), the Jahrum Formation pinches out to less than 300 m in stratigraphic thickness (Figure 4b). Here, the main lithology is medium-bedded limestone with abundant shallow- and open-marine foraminifera (e.g. Nummulites, Discocyclina and planktonic foraminifera). Towards the Coastal Fars, the Jahrum Formation carbonates grade into deeper-water shales of the Pabdeh Formation.

The considerable stratigraphic thinning of the Jahrum Formation to the south reflects an Eocene, southward-dipping, inner-ramp-to-basin transect in the study area. Evidence for this overall southward-deepening trend is also found in paleobathymetric evidence including geological evidence. The Hormuz salt diapirs (buried and plug dome; Jahani et al., 2007) complicate the stratigraphic framework due to regional changes in accommodation space overriding the southward deepening trend.

DEPOSITIONAL FACIES ASSOCIATIONS

The lower stratigraphic interval of the Jahrum Formation was altered by dolomitizing fluids. Here, the skeletal components are not readily identified and biogenic remains are generally rare to absent (Figures 5a to 5c). For this reason, we focused on the limestone portions of the Jahrum Formation. Based on sedimentological and paleontological field and thin-sections evidence, eight characteristic carbonate facies and a series of sub-facies are recognized (Table 1). Carbonate facies are divided into two main groups: autochthonous and allochthonous deposits. Allochthonous deposits of the Jahrum Formation - mainly transported and re-sedimented shallow-marine facies - will be discussed at the end of this section.

Thin-section evidence indicates that most of these sedimentary rocks are micrite supported with the groundmass being fine-grained carbonate mud. Marine and other cement phases are comparably rare and only listed where more frequent in rare instances. Due to the abundance of larger benthic foraminifera and micrite, the Jahrum Formation platform facies is referred to as “foraminifera-dominated wackestone to packstone”.

Inner-Ramp Depositional Facies

Orbitolites FaciesDescription: The main feature of this facies is the dominance of Orbitolites (Figure 5d). Other biogenic components include Coskinolina, miliolids, molluscs (mainly bivalves) and dasycladaleans algae. One sub-facies of the Orbitolites facies was defined and is listed in Table 1.

Interpretation of Depositional Environment: A lack of rotaliid forms and a high abundance, but low diversity, of miliolids and Coskinolina may be indicative of more than normal saline conditions, typical of slightly restricted inner lagoonal setting (Jorry, 2004; Beavington-Penney et al., 2006; Adabi et al., 2008; Moallemi, 2009). A low-energy environment of deposition is indicated by the high lime mud content of this facies. It seems likely that waves were degraded seawards where wave base-seafloor friction resulted in a much reduced wave climate in more proximal portions of the Jahrum depositional environment (Immenhauser, 2009). In summary, this facies reflects the most proximal portions of the Jahrum carbonate platform and crops out in the northern parts of the study area.

Alveolina FaciesDescription: This facies is characterized by an abundance of Alveolina and miliolids with common small echinoid and mollusk fragments. Other common grains include peloids, Somalina, small foraminifera (rotaliids), Orbitolites, small Nummulites, dasycladacean algae and rare small coralline algae fragments (Figures 5e to 5i, and 6a). This facies is frequently dolomitized with idiotopic euhedral dolomite crystals. Alveolina facies shows five sub-facies types (Table 1). This facies is common in the middle interval of the Jahrum Formation ranging in stratigraphic thickness between 40 to more than 100 m between different outcrops. This facies is one of the most frequent facies types of the Jahrum Formation in the northern parts of the study area.



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i

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Figure 5: Outcrop views and photomicrographs of Jahrum Formation thin sections. (a to c) Dolostone in lower portions of the Jahrum Formation. The biomouldic porosity is the result of dissolution of large benthic foraminifera. (d) Orbitolites facies. (e to i) Alveolina facies. This facies shows five sub-facies types including: (e and f) Rotalia Somalina Alveolina packstone to grainstone, (g) Rotalia miliolid Alveolina packstone to grainstone, (h) Rotalia bivalve green algae Alveolina packstone and (i) Bivalve green algaeCoskinolina Alveolina packstone.

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Interpretation of Depositional Environment: Based on the abundance of Alveolina, well -preserved miliolids, green algae and imperforate benthic foraminifera it is proposed that the sediments were deposited in protected lagoonal back-bank environments. Although abundant imperforate foraminifera can be indicative of elevated salinities, normal marine salinities are inferred due to the co-occurrence of stenohaline biota such as robust nummulitids (Beavington-Penney and Racey, 2004) and similarly, echinoids commonly prefer normal marine environments. The occurrence of soritids supports an environment within sheltered lagoonal or backshoal environments (Zohdi et al., 2007; Zamagni et al., 2008).

Coskinolina-Dictyoconus FaciesDescription: This facies is dominated by benthic foraminifera (mainly Dictyoconus and Coskinolina) and other bioclasts (e.g. uniserial foraminifera, miliolid, Valvulina, bivalve, Alveolina and Nummulites) (Figures 6b to 6d). Small peloids are common. Some of the grains have been partially micritized. This facies comprises three sub-facies types, which are listed in Table 1.

Interpretation of Depositional Environment: Valvulinids, uniserial foraminifera, miliolids, Coskinolina, Dictyoconus and other bioclasts in this facies type represent shallow lagoonal conditions with moderate water circulation above the fair-weather wave base. The presence of bivalve shells and Nummulites indicates relatively open-marine conditions with normal salinity (Michel et al., 2011; Lécuyer et al., 2012). The presence of grain-supported texture (Figure 6d), local marine calcite cements and occasional presence of wave and current structures including large-scale cross-bedding perhaps related to sand waves (Figure 6e) suggests that this facies was deposited in a higher-energy environment (Calvet and Tucker, 1988; Immenhauser, 2009). This facies is typical for periods of relative tectonic stability, as is indicated by the great lateral extent and the uniform thickness and facies of these deposits.

Miliolid Red-algae FaciesDescription: The algal limestone facies is characterized by abundant red-algae along with other bioclasts such as miliolid and porcellanous benthic foraminifera, bryozoans (Figures 6f and 6g), Nummulites, bivalve fragments and skeletal debris in a mainly micritic matrix. This facies type occurs

Table 1: Summary of the facies and sub-facies descriptions of the Jahrum Formation in the southeast Zagros Basin.

Facies Sub-facies (microfacies) Environment

Orbitolites facies 1-A. Miliolid Orbitolites packstone (Figure 5d) Inner ramp

Alveolina facies

2-A. Rotalia Somalina Alveolina packstone to grainstone (Figures 5e and f) 2-B. Rotalia miliolid Alveolina packstone to grainstone (Figure 5g) 2-C. Rotalia bivalve green algae Alveolina packstone (Figure 5h) 2-D. Bivalve green algae Coskinolina Alveolina packstone (Figure 5i) 2-E. Nummulites (large and small) Alveolina packstone (Figure 6a)

Inner ramp

Coskinolina- Dictyoconus facies

3-A. Valvulina uniserial foraminifera miliolid Coskinolina-Dictyoconus packstone to grainstone (Figure 6b) 3-B. Austrotrillin aeocaeina Coskinolina-Dictyoconus packstone (Figure 6c) 3-C. Miliolid Nummuites Coskinolina-Dictyoconus packstone to grainstone (Figure 6d)

Inner ramp

Miliolid red-algae facies

4-A. Peloid miliolid red-algae grainstone (Figure 6f) 4-B. Peloid broyzoa Nummulites red-algae packstone to grainstone (Figure 6g) Inner ramp

Nummulites facies

5-A. (A and B-forms) Nummulites packstone (Figures 6h, i and Figure 7a) 5-B. Peloid A-form Nummulites packstone (Figure 7b)

Inner to middle ramp

Nummulites - Discocyclina facies

6-A. Nummulites-elongate Discocyclina wackestone to packstone (Figure 7c) 6-B. Nummulites Operculina packstone (Figure 7d) Middle ramp

Lime mudstone facies 7-A. Planktonic mudstone (Figure 7e) Outer ramp

Re-sedimented deposits

Middle to outer ramp



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i

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Figure 6: (a) Nummulites (large and small) Alveolina packstone belonging to Alveolina facies. (b to d) Coskinolina-Dictyoconus facies consists of three sub-facies: (b) Valvulina uniserial foraminifera miliolid Coskinolina-Dictyoconus packstone to grainstone, (c) Austrotrillina eocaeina Coskinolina-Dictyoconus packstone, and (d) Miliolid Nummulites Coskinolina-Dictyoconus packstone-to-grainstone, deposited in moderate to high-energy settings. (e) Large-scale cross-bedding in Coskinolina-Dictyoconus facies (height of section is 12 m). (f and g) Miliolid red-algae facies includes two sub-facies: (f)

Peloid-miliolid red-algae grainstone, and (g) Peloid broyzoa Nummulites red-algae packstone tograinstone. (h and i) (A and B-forms) Nummulites packstone belonging to Nummulites facies.

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in all sections of Jahrum Formation in the southeast Zagros Basin and passes stratigraphically into Coskinolina-Dictyoconus facies. This facies is common in the upper portions of the Jahrum Formation and occurs in the northern and as well as southern outcrops. The algal facies is dominated by calcareous red-algae and comprises two sub-facies types (Table 1).

Interpretation of Depositional Environment: The biota and the dominance of packstones and grainstones indicate shallow-water, comparably higher-energy conditions. True reefal structures, here defined as wave-resistant, bioconstructed framework rising above the carbonate seafloor, are not found. The algal and foraminiferal algal packstones represent distal parts of the inner ramp settings, above the fair-weather wave base. Winnowing was sufficient to remove the micrite matrix and the bioclasts reflect both the in-situ ramp benthos (calcareous red-algae and foraminifera) and extraclasts that were entrained by currents and wave orbitals and re-deposited in this setting.

Miliolid red-algae facies are interbedded with Coskinolina-Dictyoconus and Nummulites facies. The presence of marine calcite cements occluding pore space and the good sorting of the components indicates higher hydrodynamic conditions (Taheri et al., 2008). Most peloids in this facies are interpreted as micritized fragments of red-algae or perhaps smaller benthic foraminifera (e.g. miliolids).

Middle-Ramp Depositional Facies

Nummulites FaciesDescription: Nummulites (Nummulites perforatus) facies is characterized by accumulations of (A and B-forms) Nummulites occurring as packstone and rarely wackestone (Figures 6h, 6i, 7a and 7b). Sedimentary structures (e.g. Nummulites imbrication and large-scale cross bedding) are rare. Nummulites foraminifera are very well preserved and both generations are present, though the assemblage is dominated by the larger B-forms. Taphonomic study of the tests suggests that these accumulated largely in-situ. Other bioclasts include Discocyclina, Operculina, echinoids and bivalves. Foraminifera show variable degrees of imbrication (Figure 7a). The main body of the unit passes upsection into Nummulites-Discocyclina facies. The Nummulites facies is widespread and characteristic of the Jahrum Formation. This facies mainly occurs in the lower to middle portions of southern outcrops in the Khush and Genow anticlines. The Nummulites facies can be divided into two sub-facies (Table 1).

Interpretation of Depositional Environment: Nummulitid foraminifera indicate tropical and subtropical shallow-water environments, normal seawater salinity and a fairly high rate of sedimentation (Beavington-Penney and Racey, 2004; Nadjafi et al., 2004; Jorry et al., 2006; Guido et al., 2011; Höntzsch et al., 2011; Mateu-Vicens et al., 2012).The large diameters, the nature and sorting of the Nummulites tests and grain-supported texture of the A and B-forms Nummulites packstone are all indicative of a moderate bank environment. This facies represents the development of Nummulites banks in a middle ramp setting and was most likely deposited below the fair-weather wave base. Evidence for this comes from the lack of abrasion of foraminifera tests. This feature is generally considered to indicate autochthonous accumulations, winnowed only by rather weak currents, similar to the environment of deposition interpreted for Eocene foraminiferal banks in Oman and Egypt (Aigner, 1985; Racey, 2001). Nummulitic banks (‘Buil level’) occur within the middlemost composite sequence of the southern outcrops (e.g. Genow and Khush anticlines).

Nummulites-Discocyclina FaciesDescription: This foraminiferal facies is chiefly composed of intensely bioturbated packstone with local wackestone, characterized by a diverse foraminiferal assemblage dominated by Nummulites, Discocyclina and Operculina (Figures 7c and 7d). Other bioclasts include Orbitolites, Alveolina, echinoids and bivalve shell fragments. Locally, packstones are developed with tightly packed, flat Discocyclina and Operculina tests, which are variably imbricated (Figure 7d). This facies is subdivided into two sub-facies types (Table 1). Nummulites-Discocyclina facies is present in the southern outcrops of the Khush and Genow anticlines only.



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Figure 7: (a) Imbrication of discoidal bioclasts (e.g. nummulitids); (b) Peloid A-form Nummulites packstone. (c and d) Nummulites-Discocyclina facies forms the distal part of the middle ramp and is associated with lime mudstone facies. (e) Lime mudstone facies. (f to h) Reworked material includes platform top carbonate clasts and shallow-water bioclasts. (i) Nummulithoclastic debris and allochthonous bioclast in lower JahrumFormation.

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Interpretation of Depositional Environment: Abundant thin Discocyclina and the abundance of micritic matrix suggests that these deposits formed in a low- to moderate-energy, fore-bank environment (Jorry et al., 2003; Afzal et al., 2011; Jach et al., 2011; Misra et al., 2011). Fragmented Nummulites are interpreted to have been reworked and then transported from higher-energy areas. The interbedded stratigraphic nature of this facies type with the lime mudstone facies may also suggest that these two facies were deposited in adjacent and interfingering environments.

According to Geel (2000), as depth increases, flatter, large perforate and planktonic foraminifers predominate. In the Jahrum Formation carbonate ramp, the replacement of thick Nummulites specimens by flat and thin morphotypes along with Discocyclina and Operculina supports the previous statement.

Outer-Ramp Depositional Facies

Lime Mudstone FaciesDescription: The lime mudstone facies comprises mudstones and marls with planktonic foraminifera (Figure 7e) and a sparse, low-diversity benthic assemblage dominated by flat Discocyclina, Operculina, Nummulites and bivalves. The regional distribution of this facies type is limited to the southern outcrops and predominantly the Khush anticline. One sub-facies type is shown in Table 1.

Interpretation of the Depositional Environment: The horizontal bedding style and the absence of cross-bedding or similar wave and current induced structures indicate that this facies was deposited in a low-energy environment. The dominance of fine-grained sediments and the lack of abraded detritus indicate a low-energy depositional environment. The presence of planktonic foraminifera and the benthic assemblage, indicative of relatively deep-water, outer-ramp conditions (Sadeghi et al., 2011), confirm this interpretation. The presence of barren horizons and the preservation of organic detritus are probably suggesting periodic, regionally limited bottom water anoxia, though the general presence of benthic organisms indicates that the water masses were usually oxygenated.

Re-sedimented DepositsDescription: These layers contain abundant reworked material including transported shallow-water bioclasts and abundant coarse carbonate breccia clasts embedded in lime mudstone facies with abundant planktonic foraminifera. Individual reworked clasts reach dimensions of 20 cm to more than 1 m (Figures 7f to 7h). Bioclasts (e.g. Nummulites) within the layers are characterized by a wide range of foraminifera test-damage features (commonly referred to as nummulitho-clastic debris), ranging from relatively minor to severe, including breakage of the marginal cords of the penultimate (and younger) whorls and fracturing through the entire test thickness (Figure 7i). These re-sedimented deposits are found in the lower intervals of the Jahrum Formation and mainly so in the southern outcrop belts. This facies type is also characterized by irregular, non-planar bedding geometries with common basal scours and lateral variations in bed thickness (from 0.2 to more than 1.5 m) (Figures 8 and 9a to 9c).

Interpretation of Depositional Environment: The abundance of platform-top carbonate clasts and shallow benthonic foraminiferal taxa (miliolids and discorbids), often associated with sea grass meadows (e.g. Brasier, 1975; Mateu-Vicens et al., 2010) along with planktonic taxa and large benthic foraminifera, such as Operculina and Discocyclina, indicate transport and re-sedimentation of components from different bathymetric zones and ramp environments. Elsewhere, re-sedimented Eocene large benthic foraminifera, have been described from a diverse array of gravity flows, and all of these are commonly related to platform-margin collapse and deposited most often in centimeter to meter-thick turbidite beds (e.g. Eocene deposits from Spain; e.g. Payros et al., 1999; Geel, 2000).

With respect to the reworked clasts of shallow-water facies, two main lines of interpretation offer themselves. One suggests that these deposits might be the result of allochthonous debris flows deposited stratigraphically directly above the Pabdeh Formation. Here, perhaps the most significant aspect is that carbonate breccias consist nearly exclusively of shallow bioclast inner-shelf limestone facies. Similar features were described from Upper Cretaceous limestones of Albania (Rubert et al., 2012) with brecciated limestones intercalated with debris flow deposits and thick slumped levels.Copyright Gulf PetroLink 2016. All Rights Reserved. Downloaded by [email protected] IP:41.239.122.32


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Figure 8: Key to Figures 9 through 16.

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This observation contrasts to the second possible interpretation of these clasts, namely the in-situ formation of breccias in the wave attenuation zone. Séguret et al. (2001) describe lime mudstones and calcarenites grading vertically and laterally into in-situ brecciated facies. According to these authors, these breccia deposits resulted from deformation and liquefaction of semi-lithified sediments in the wave attenuation zone, i.e. the zone where the basinward sloping carbonate seafloor intersects with the storm wave base. Due to the wave orbital-seafloor interaction, in-situ brecciation of semi-lithified sedimentary rocks took place. We here suggest that this mechanism is not applicable because it would not explain why the brecciated facies is mainly allochthonous in nature.

SPATIAL AND TEMPORAL CHANGES OF PLATFORM ARCHITECTURE

The Jahrum Formation carbonate platform was attached along the northern margin of the Arabian Plate. Stratigraphic and sedimentological analyses of studied exposures (Figures 10 to 13), as well as the lateral and vertical facies distributions observed in the cliff photomosaics, suggest that the depositional profile is consistent with a carbonate ramp system. Inner-ramp settings are characterized by alveolinid-dominated facies types, middle ramp settings are dominated by nummulitids and outer-ramp settings are prevailed with lime mudstone facies and re-sedimented deposits.

In terms of stratigraphic age, spatial facies distribution and depositional environments, the evolution of the Eocene carbonate ramp in the southeast Zagros Basin is subdivided into two main stages (Figure 14): (1) a Lower Bartonian distally steepened carbonate ramp, and (2) a Middle to Upper Bartonian homoclinal carbonate ramp.

Lower Bartonian Distally Steepened Carbonate Ramp

During the Early Bartonian, the architecture of the Jahrum Formation ramp is indicative of a distally-deepening relief. In the northern outcrop belts, the facies consists of pack- to grainstone, with abundant porcellanous large benthic foraminifera (e.g. Alveolina, Somalina, Orbitolites, Coskinolina and Dictyoconous), miliolids, green algae and bivalves. The biotic assemblage suggests that these


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successions formed in water depths of less than 40 m (Beavington-Penney and Racey, 2004; Huyghe et al., 2012), with perennial seagrasses and other macrophytes with flattened blades (Astibia et al., 2010; Mateu-Vicens et al., 2010) (Figures 10 and 11). In southern outcrop belts, the Jahrum Formation is more commonly characterized by biota such as alveolinids, miliolids, Nummulites, Discocyclina, Operculina, planktonic foraminifera, bivalves, echinoids and red-algae (Figures 12 and 13). These fossils have different ecological preferences.

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Figure 9: (a to c) Channelized systems and their associated geometry (erosive surfaces, lenticular/sigmoidally-shaped deposits and lateral accretion features) in lower parts of Jahrum Formation. The shoal-water facies filling channel elements differs markedly from the encasinghemi-pelagic facies. LBF = large benthic foraminifera.



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Figure 13: Stratigraphic facies distribution and sequences of Jahrum Formation at the Khush Anticline showing an overall regressive trend. Here,dolomitization of the lower Jahrum did not take place.

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Some, such as miliolids and Alveolina, live in the inner ramp, whereas large flat Discocyclina, Operculina and planktonic foraminifera are observed in deeper environments, at the lower limit of the photic zone (Luterbacher, 1998). This contradictory live mode suggests transport and re-sedimentation of facies types.

The presence of abundant inner-ramp carbonate facies in more external ramp domains suggests the possibility of a distally steepened margin with major downslope reworking below the photic zone (e.g. Read, 1985; Burchette and Wright, 1992). Distally steepened ramps are typified by relatively high rates of carbonate sediment production along their outer fringes, which results in coarse-grained sediment



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accumulations (Burchette and Wright, 1992; Pomar, 2001). Moreover, the presence of turbiditiy flows, mainly channel structures, and reworked inner platform facies strongly supports the notion of a distally steepened ramp and point to tectonic and/or diapiric instabilities. Interestingly, these mass flow deposits are not present in the middle Bartonian deposits of the Jahrum Formation. We thus suggest that the overall ramp geometry changed gradually from Early to Middle Bartonian times.

Middle Bartonian Homoclinal Carbonate Ramp with Nummulitic Banks

In the Middle Bartonian, petrographic analyses and facies associations are indicative of a more gradual deepening trend from the shallow-ramp interior into the basin (Figure 14). This is more compatible with a homoclinal-ramp setting rather than a shelf or distally steepened ramp. No evidence of mass-transport sedimentation is found in Middle Bartonian portions of the studied sections. Micritic matrix is significant in all facies types and there is no evidence for reefal framestone or grainstone belts in the mid- to distal ramp. All of the above arguments support the notion of a homoclinal carbonate ramp architecture.

The middle ramp is characterized by Nummulites accumulations typical for homoclinal, low-angle ramps of this age as noted by Jorry et al. (2003) and Jorry (2004). Nummulites banks, characterized by a mono-specific association, separate a restricted area (back-bank setting) from an open-marine zone (fore-bank settings). The back-bank deposits contain highly robust forms, often associated with mollusks (bivalves and gastropods) and porcelaneous foraminifera (miliolids, Alveolina and Orbitolites); and the fore-bank is characterized by large flat forms of Nummulites associated with large, flat rotaliids such as Discocyclina.

Upper Bartonian Carbonate Ramp

In the Late Bartonian, the Jahrum Formation carbonate rocks in all studied outcrops are typified by larger benthic foraminifera (e.g. Orbitolites), along with gastropods, bivalve and green algae all pointing to very shallow water. This overall pattern is more compatible with a proximal inner ramp setting both in northern and southern outcrops (Figure 14).

Middle Eocene–Early Oligocene Transition between the Jahrum and Razak Formations

The transition of the Jahrum Formation carbonate ramp to the Lower Oligocene mixed clastic-carbonate Razak Formation in the southeast Zagros Basin has been traditionally accounted by an increase of tectonic activity and fall in relative sea level combined with increased terrigenous input (Figures 13 and 14) (Alavi, 2004; Zohdi et al., 2011). The transition from a carbonate ramp platform of the Jahrum Formation to the mixed carbonate-siliciclastic shelf of the Razak Formation is here considered as the result of a continuous decrease in accommodation space in the outer-ramp margin as a function of relative sea-level change (Figures 3 and 10 to 13). This stratigraphic level is close to the Eocene/Oligocene Boundary at which a eustatic sea-level fall has been proposed by Haq et al. (1987). Elsewhere, there is no evidence for a Late Eocene depositional age such as found in many other localities on the Arabian Plate (e.g. Biladi-1 Well in North Oman, Abu Dhabi, Yemen and Saudi Arabia; Hughes Clarke, 1988; Brannan et al., 1999; Sharland et al., 2001).

SEQUENCE-STRATIGRAPHIC FRAMEWORK

The sequence-stratigraphic framework of the Jahrum Formation in the southeast Zagros Basin has not yet been in the focus of a modern sequence-stratigraphic study. We here present a tentative first model.

‘Second-Order’ Depositional Sequence: The overall sequence-stratigraphic pattern of the Jahrum Formation carbonates is perhaps best interpreted as a large-scale regressive unit (highstand system tract). Arguments include the stratigraphically underlying distal marine shale deposits of the Pabdeh Formation and the overlying carbonate-terrigenous shallow-marine to continental deposits of the Razak


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Shallow-marine carbonatePure dolostone

Figure 14: Time-slice sketches (Ypresian Jahrum to Rupelian Razak formations) of the proposed paleoenvironmental and architectural evolution of the Jahrum Formation. The proposed location of different facies types is indicated. For key to symbols please refer to Figure 8. Note change from distally steepened ramp (Ypresian to Early Bartonian) to homoclinal ramp (Middle Bartonian to Rupelian). Relative position of studied anticline sections is indicated.

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Mixed carbonate-siliciclastic shelf. Razak Formation deposits signal increase of tectonic activity and fall in relative sea level combined withincreased terrigenous input.

Proximal inner ramp setting in northernand southern outcrops.Very shallow water biotic assemblage.

Homoclinal ramp. No evidence of mass transport, no reefalor grainstone belts, abundant micrite.

Nummulites banks, characterized by a mono-specific association, separate a restricted area (back-bank setting) from anopen-marine zone (fore-bank settings).

Distally steepened ramp. Shallow-water (< 40 m) biotic assemblagein northern outcrop belts.

Mixed biotic assemblage in southern outcrop belts suggests gradual oversteepening which resulted in transport and re-sedimentation along the distallysteepened ramp.

Distally steepened carbonate ramp. Shallow-marine carbonate.Pure dolostone.

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Formation (Figures 10 to 13). Comparing our findings with the Arabian Plate conceptual framework published in Sharland et al. (2001) the Jahrum Formation in the southeast Zagros Basin coincides with a second highstand system tract of the megasequence AP10 (Early Paleocene to Latest Eocene).

‘Third-Order’ Depositional Sequences: When focusing on the Jahrum Formation limestone successions in the study area, a further, higher-order pattern is obvious (Figures 10 to 13). In all studied outcrops, depositional sequences reflect a dominantly retrogradational stacking pattern prior to the formation of a maximum flooding interval (MFI), and a subsequent progradational and/or aggradational stacking geometry that accumulated during a relative sea-level highstand near the top of the succession.

Maximum Flooding Interval (MFI): In the Jahrum Formation outcrops, evidence for a specific, regionally significant maximum flooding surface (i.e. a well-defined marine hardground surface) is lacking. Nevertheless, there is a clear physical expression of the facies change between the transgressive and the highstand systems tracts in outcrops. We tentatively assign the interval of maximum flooding to this transition. Specifically, in the southern outcrop belt, the maximum flooding interval is placed at the end of the transgressive systems tract and of the onset of the planktonic mudstone sub-facies (Table 1). In the northern outcrop belts, the MFI is placed at the boundary between peloid bryozoan Nummulites red-algae packstone beneath and the grainstone sub-facies above (Table 1). Refer to Figures 10 and 11 for an overview.

Transgressive Systems Tract: The transgressive system tract is recorded from all of the studied outcrops of Jahrum Formation (Figures 10 to 13). The transgressive systems tract is built up of Nummulites-Discocyclina facies, Nummulites facies and in northern outcrops Alveolina facies with abundant rotalia. This systems tract is restricted by the maximum flooding interval at the top with the planktonic mudstone at the southern outcrops and peloid bryozoan Nummulites red-algae packstone to grainstone sub-facies in the northern outcrops. In southern outcrops (e.g. Khush), the re-sedimented carbonate deposits in lower portions of the Jahrum Formation are restricted to the base level of this package and correspond to a conformable, but relatively sharp sequence boundary between Pabdeh and Jahrum formations during a drop in the relative sea-level.

Highstand Systems Tract: The highstand system tract is identified from all of the studied outcrops. The highstand systems tract is bounded by the maximum flooding interval at the base and a clear sequence boundary (SB1) at the topmost part of the Jahrum Formation (Figures 10 to 13). The open-marine pelagic facies in southern outcrops and peloid bryozoan Nummulites red-algae packstone to grainstone in northern outcrops followed by a gradual shallowing-up sequence. This progradation is the main criterion for interpreting deposition as a highstand system tract. It is capped with Orbitolites facies, Coskinolina-Dictyoconus facies and miliolid red-algae facies which indicate high-frequency changes of the relative sea level.

This transgressive and highstand system tract coincide with third-order cycles of the upper Middle Eocene (Bartonian) as suggested by Haq et al. (1988).

Sequence Boundary: Based on facies and paleoenvironment interpretation, the regional sequence boundary is placed in all studied sections at the transition between the Jahrum carbonate Formation and the Razak Formation composed of mixed siliciclastic-carbonate sedimentary rocks. Specifically, at outcrops the sequence boundary is an erosional surface with an irregular relief that separates marine carbonate rocks of the Middle Eocene Jahrum Formation from alternating continental and marine successions of the Razak Formation (Early Oligocene) (Figures 10 to 13). This stratigraphic level is close to the Eocene/Oligocene boundary at which a eustatic sea-level fall has been proposed by previous workers (Haq et al., 1987; Zachos et al., 2001; Sharland et al., 2001; Miller et al., 2005).

The conformable, but stratigraphically well-defined increase in mass-transport deposits at the base of the Jahrum Formation in the southern anticlinal outcrops corresponds, in the view of the authors, to a sequence boundary between the Pabdeh and Jahrum formations. Evidence for this comes from


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significant variations in the lithologic characteristics (sharp change from marine shale to limestone) and also the fossil content. This boundary is the product of a drop in relative sea level and considered as a correlative basinward conformity between the Pabdeh and Jahrum formations.

Correlation with Arabian Plate Sequence Stratigraphy

According to Sharland et al. (2001), one depositional megasequence (AP10) has been identified across the Arabian Plate during the Early Paleocene to latest Eocene (63–34 Ma). Assuming that the eustatic curve of Haq et al. (1988) is a more-or-less reasonable approximation of global sea-level change during this time, the AP10 megasequence represents two ‘second-order’ depositional sequences sensu Vail et al. (1977). During the 29 Myr comprised in megasequence AP10, two maximum flooding “surfaces” and two depositional cycles mainly formed by shallow-water carbonates of the Umm Er Radhuma and Dammam formations have been identified (Sharland et al., 2001).

The studied sections of the Jahrum Formation in the southeastern Zagros Basin coincide with the second highstand system tract contained in AP10. The regional pre-Neogene unconformity at the top of this megasequence (AP10) is coincident with the unconformity between the top of the Jahrum and the base of the Razak formations as discussed before.

ORIGIN OF RE-SEDIMENTED DEPOSITS

The nature and mechanism of re-sedimented deposits in the southern Jahrum exposures (e.g. Khush Anticline) is an unsettled issue. The stratigraphic units that yield these deposits coincide in time variously with phases of a tectonic over-steepening of the basin margin or episodes of relative sea-level fall. In this section, we discuss the postulated triggers.

Towards the south, i.e. the basin, the Jahrum Formation ramp deposits gradually thin and then laterally grade into the Pabdeh Formation (Figure 15). In the southern outcrops (e.g. Khush Anticline, Figures 13 and 15), the Jahrum Formation contains limestone and deposited at the platform outer ramp edge (Figure 16). Spence and Tucker (1997) suggested, that these physiographic portions of carbonate systems are very sensitive to destabilization processes, which might result in the generation of re-sedimented carbonate deposits. These authors invoked hydrostatic overpressure, occurring during periods of relative sea-level fall at discrete, hydrologically confined horizons beneath the sea floor as one of the destabilization mechanisms. Furthermore, evidence is found that the stress in the sedimentary packages increases near the platform edge- or the outer ramp- and the upper slope as pore-fluids drain from the sediment during a sea-level fall. Obviously, these mechanisms will be more significant during periods of high-rate sea-level change. Similarly, during sea-level fall, the depth of the storm wave base is lowered and affects previously undisturbed slope and outer ramp areas. In southern outcrop belts, re-sedimentation clearly coincides with a period of sea-level fall as expressed in a correlative, basinward conformity between Pabdeh deep-marine shale and Jahrum carbonates. Clear evidence for regional, extended subaerial exposure (e.g. siliciclastic flux, erosional incisions and sedimentary infills) is lacking. We thus conclude that sea level was lowered but not to a degree that would have exposed the Jahrum inner to middle ramp settings.

Other potentially significant processes triggering ramp instability and mass transport of carbonate deposits include periods of tectonic activity in foreland-basin settings with associated forebulge steepening and seismic activity (Payros and Pujalte, 2008). The southeastern Zagros Basin forms part of a foreland margin platform. Locally the stratigraphy is pierced by Hormuz salt diapirs. Hormuz salt is present in more than 200 domes distributed throughout the southeastern part of the Zagros Fold-Thrust belt (Jackson and Talbot, 1991; Jahani et al., 2007; Figure 17).

The Hormuz series consists of a multi-color melange of salt, anhydrite, black dolomite, shale, red siltstone and sandstone. Also present are some metamorphic and volcanic blocks. These are interpreted either as allochthonous basement clasts (Kent, 1979) or as syn-Hormuz deposits, transported to the surface by diapirs. The Hormuz and equivalent series were deposited in an evaporitic basin during the Late Neoproterozoic–Early Cambrian (Motiei, 1993). Salt diapirs in the eastern Zagros Basin



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present a large variety of morphologies, from high relief (see Figure 4 in Jahani et al., 2007), actively upward-moving diapirs, to entirely eroded and inert structures. In addition, diapirs display variable dimensions at outcrop scale, ranging from craters a few kms wide to diapirs and salt glaciers more than 15 km wide.

Salt diapirs can either reach the surface or only dome-up the overlying strata without reaching the surface. Most previous workers (Sherkati and Letouzey, 2004; Jahani et al., 2009) agree that all, or almost all, salt diapirs of the eastern Fars were active before the orogeny. The Zagros Orogeny during the Cenozoic may have had an accelerating effect on Hormuz salt halokinetics and many Hormuz


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54°E 55° 56°

54° 55° 56°

28°

27°

28°N

27°

Bnaru Gach

East Mazyian(Namak)

Kureden

Nesf rah

Gach

Burkh(Siah Taq)

ShurChah-Banu(Hormuz)

Qaleu-Shur

Bavosh

Gavbast Bastak 2

Bastak 1

GazehKameshk

Kalat

Namaki(Darbast

or Taranjeh)Charak

(Bavardin)

Mughu

Chah-Musaleh(musallem)

Champeh

Zendan

Moallem

Bostaneh

Homiran(Berke-e-suflin)

Namakdan

Hengam

Gaverzin Larak

Herang(Chahar-Berkeh)

Baviun

Bam

Shu

Shamilo

HarmandanKhain

Tashkend(West Baz)

Tarbu (Baz)

Muran-WFinu

Jaghu 2

Jaghu 1(Khushk

Kuh) Darbast(Shamil,Kushk)

Jalabi

Handon(Ardan)

Khorgo

Muran(Tang-E-Zagh)

Faraghun

Study Area

Palangi

J?-K?-Pa-Eo-O1

Eo-O1K3?

O1-M O1

O1-M1M1,2

K3-M

Paskhand

Parak

South Parak M2

M3-P1

K3

O1-M

= Pliocene= Late Miocene= Middle Miocene= Early Miocene= Oligocene= Eocene

PlM3M2M1O1Eo

= Late Cretaceous= Middle Cretaceous= Early Cretaceous= Jurassic= Triassic

= PaleoceneK3K2K1

JTr

Pa

O1,-M2

K1-J? Zangard

M1

O1-M1,2O1-M

C

O1O1 and K3

M-P1 M-P1

M

M2

M1,2

M-P1

C3-M2

K3-Eo

M-P1

Eo-O1-MK3-Pa

Do-Ao

Ilcheh

MijunKhamir

Pol

Anguru(Gasho)

Guniz(Gurdu-Siah)

Kalat Bala

Ginau

Hurmuz

Gachin(Suru)

M3-P1

M1,2-P1 M3-P1M1,2

M2-3

M2-P1

M2-3

C2,3-M3-p1

O1-M1

M3-P1

O1-M

M?

M

Deh-Kuieh

Last reactivation before Zagros folding

Last reactivation during Zagros folding

Salt diapirs with no evidence of thinning

Buried domes

N

km

400

Figure 17: Distribution and age of salt diapirism in the southeast Zagros Basin (modified after Jahani et al., 2007).

Qheshm IslandWell Hulur-01

SouthernKhush Outcrop

To Basin

NorthernFaraghun Outcrop

A A’

Bar

toni

an

Pab

deh

Pab

deh

Jahr

um

Ypre

sian

toLu

tetia

n

Pabdehshale

Pre-Neogene major regional unconfromity sensu Vail et al. (1977)

Jahrumdolostone

Jahrumlimestone

Jahrumlimestone

Figure 16: Correlation of the limestone portions of the Jahrum Formation across the study area (see Figure 15 for location) and into the basinal Pabdeh Formation.

Jahr

um

100

0 m

10 km

Southern Khush Outcrop Northern Faraghun Outcrop



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salt domes that were close to the surface during the Paleozoic and Mesozoic surfaced during the orogeny. The movement of salt diapirs during the orogeny had a pronounced effect on the subsequent deposition of the Eocene successions in the southeast Zagros Basin (Jahani et al., 2009).

During the deposition of the Jahrum Formation, salt diapirs moved up into the zone of shallow-marine carbonate production (Jahani et al., 2009; Figures 18a to 18c). Jahrum carbonate stratigraphies are up to 600 m thick and most exposed successions commonly shallow upwards. Geological observations made included thickness variations and development of growth strata adjacent to the salt plugs, and the presence of recycled Hormuz materials (Figure 18c) in the adjacent sedimentary successions. Jahrum platforms were growing during salt movement, and thus had their stratigraphy and platform

a b

c

Salt plugSalt plug

Jahrum

Jahrum

Jahrum

Recycled Hormuz debris

Jahrum

West East West East

Pabdeh

Figure 18: (a and b) Photographs of salt plug in the vicinity of Jahrum Formation in the study area. 18a, the field view of shrubs is approximately 50 cm length, and 18b, the field of view of the salt plug is approximately 1,500 m wide. (c) Well-rounded pebbles of recycled Hormuz debris in theJahrum strata, showing immature grains of mixed sizes.


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development controlled by the salt diapirism. Considering these circumstances, it seems likely, that the Jahrum steepened ramp was faulted and brecciated by salt movement resulting in downslope gravitational movement of consolidated and unconsolidated carbonates.

In conclusion of the above discussion, the combined effects of sea-level fluctuations and tectonic activity, including active faulting and salt diapiric movement during the deposition of the Jahrum Formation, probably resulted in the initiation of mass movements and, more generally, affected the overall ramp geometry of the Jahrum Formation in Iran.

CONCLUSIONS

The Eocene Jahrum Formation in the southeastern Zagros Basin of Iran is among the most spectacular, and probably most controversial, units of the Zagros Basin. Here we present the first modern and detailed sedimentological, paleoecological and sequence-stratigraphic interpretation of the Jahrum Formation. This work challenges previous, interpretations based mainly on data from the southwest Zagros Basin (Dezful Embayment and Fars Region).

Based on sedimentological and paleontological field and thin-sections evidence, eight characteristic carbonate facies and a series of sub-facies are recognized. Carbonate facies are divided into two main groups: autochthonous and allochthonous deposits.

The compositional analysis within a paleoecological context of the facies reveals two different sediment factories. In shallow-water settings, the euphotic factory (seagrass meadows) has been inferred, indirectly, based on component spectra (epiphytic foraminifera) that were transported and accumulated in deeper settings. Basinward, a shallow mesophotic factory has been inferred based on the prolific production of Nummulites and other larger benthnic foraminifera.

The lower Jahrum Formation is composed of a shallow-water, distally steeped carbonate ramp deposit, whose main facies can be interpreted as having accumulated in inner- to outer-ramp settings. The presence of abundant mass transport deposits in more distal sections provides evidence for the distally steepened geometry. The lack of frame builders, combined with minimal reworking of photic zone components into bathyal environments, is most likely consistent with the overall pattern of a low-angle, relatively low-energy carbonate ramp with no significant slope break for the upper parts of the Jahrum.

Comparing our findings with the Arabian Plate sequence-stratigraphic conceptual framework published in Sharland et al. (2001) the Jahrum Formation in the southeast Zagros Basin coincides with a second highstand system tract of the megasequence AP10 (Early Paleocene to Latest Eocene). When focusing on the Jahrum Formation limestone successions in the study area, a further, higher-order pattern is obvious, which coincide with a retrograded and then prograded stacking pattern, probably due to third-order relative sea-level changes. The regional sub-Neogene unconformity at the top of this megasequence (AP10) is coincident with the unconformity between the top of the Jahrum and the base of the Razak formations in the southeast Zagros Basin.

ACKNOWLEDGEMENTS

We would like to thank Dr. Alireza Piryaei from the National Iranian Oil Company (NIOC) for his valuable comments and informative discussions. Dr. Andrew Racey from British Gas Group (BG) and Dr. Mohamed Boukhari from Ain Shams University are gratefully acknowledged for benthic foraminifera determination. J. Rabani, M.A. Salehi, M. Shiri and M. Khodavaisi (from Ferdowsi University of Mashhad) are thanked for their valuable help during two months fieldwork in southeast Zagros. We thank the Research Institute of Petroleum Industry (RIPI) for permission to publish this paper. Constructive reviews by two GeoArabia reviewers greatly benefited the manuscript. GeoArabia’s Assistant Editor Kathy Breining is thanked for proofreading the manuscript, and GeoArabia’s Production Co-manager, Nestor “Nino” Buhay IV, for designing the paper for press.



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Zohdi, A., M.H. Adabi and A. Ghobishavi 2007. Palaeoenviromental reconstruction, sequence stratigraphy and palaeotemperature estimation of the Upper Paleocene to Middle Eocene Tale-Zang Formation in the Zagros Basin, (south-west Iran). 13th Bathurst Meeting of Carbonate Sedimentologists, Abstract, p. 55.

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ABOUT THE AUTHORS

Afshin Zohdi is presently a PhD student at the Ferdowsi University of Mashhad, Iran, and a Visiting Scientist at the Ruhr University Bochum, Germany. He won the first prize in the open PhD entrance exam at University of Mashhad, Iran, in 2008. He holds a BSc (Honors) in Geology from Damghan University, Iran, in 2004 as well as his MSc (top) in Geology (carbonate sedimentary rocks) in 2007 from Shahid Beheshti University in Tehran. In 2011, he received a grant from the International Association of Sedimentology (IAS) in order to attend at the 14th Bathurst Meeting of Carbonate Sedimentology in Bristol, England. His research interests include carbonate sedimentology and stratigraphy of the Paleogene, sequence stratigraphy, carbonate petrography, diagenesis and dolomitization. Afshin’s publications include studies on the sedimentary environment, sequence stratigraphy and diagenesis of the Early Paleogene successions in the Zagros and Alborz basins. He is an active member of Geological Society of Iran, Sedimentological Society of Iran and IAS.

[email protected]

Reza Moussavi-Harami is Professor of Geology in the Department of Geology, Faculty of Sciences, Ferdowsi University of Mashhad, Iran. He received his BSc in Geology from Mashhad University (1972), MSc from the University of Oklahoma, USA (1977) and PhD from the University of Iowa, USA (1980). From 1975 through 1980, he consulted for the Oklahoma and Kansas Geological Surveys. From 1980 through 1983, he was a Senior Sedimentologist with the Exploration Directorate, NIOC, Tehran, Iran. From 1994–1997, he was a Visiting Professor to the Department of Geology, University of Adelaide and consultant to the Oil, Gas and Coal Division of Mines and Energy South Australia. From 1998–2000, he was President of the Geological Society of Iran. From 1999–2001, he served as a Chairman of Geology Department at the Ferdowsi University of Mashhad, Iran. From 2000–2002, he was an Adjunct Professor to the Department of Geosciences at the University of Iowa, USA. He is consultant to RIPI, Tehran. From 2011 to present he is President of the Sedimentological Society of Iran. His main research involves basin analysis, sequence stratigraphy and the reconstruction of paleoenvironment in relation to oil and gas exploration and production. He is a member of AAPG, SEPM and EAGE.

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Seyed Ali Moallemi is Assistant Professor and Deputy Dean of Faculty Research and Development in Upstream Petroleum Industry Research Institute of Petroleum Industry (RIPI), Iran. Ali completed his PhD in Sedimentary Geology at the Shahid Beheshti University, Tehran, Iran. He has worked for more than 21 years in research, industry and consulting in Iran. He received his BSc and MSc in Geology from the University of Zahedan, Iran, in 1988 and Islamic Azad University, Iran, in 1984, respectively. Ali has authored and co-authored 25 research papers published in national and international journals. His research interests are petroleum geology, sequence stratigraphy, carbonate diageneses and reservoir geology. He is an active member of AAPG, EAGE, and Geological Society of Iran.

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Zohdi et al.

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Asadollah Mahboubi is a Professor at the Geology Department of Ferdowsi University of Mashhad in Iran since about 25 years. He received his BSc in Geology from Ferdowsi University of Mashhad in 1984, and his MSc and PhD in Geology (sedimentology and sedimentary petrology) in 1991 and 2000 from Kharazmi University in Tehran, Iran, respectively. He was previously a researcher at Research Center of National Iranian Oil Company (NIOC) in Tehran for two years. His main research interests include carbonate petrology, depositional environments and sequence stratigraphy. He has published more than 70 papers in journals and presented many papers in congresses.

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Adrian Immenhauser is a Full Professor for Sediment and Isotope Geology at the Ruhr-University Bochum, Germany. He holds a PhD in Geology from the University of Berne, Switzerland. Adrian has a longstanding interest in the geology of Oman that goes back to 1990 when he commenced fieldwork as a PhD and later as a post doc on Masirah Island and in northeast Oman. Subsequently, Adrian spent 10 years as Assistant Professor at the Vrije Universiteit Amsterdam, The Netherlands, focusing on carbonate diagenesis. Presently, his research team applies conventional and nonconventional isotope systems and other geochemical and optical tools to various carbonate materials in order to resolve the diagenetic history of reservoir units. A special focus is on the evolution and impact of discontinuity surfaces in shoalwater settings that may represent flow conduits or seals resulting in reservoir compartmentalization.

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Manuscript submitted December 16, 2012

Revised February 21, 2013

Accepted February 26, 2013


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